FIG. 1 illustrates a layout diagram of a typical pad and trace line routing scheme for a printed circuit board. The spherical input-output pads, such as a first input-output pad, may each have a trace line connecting to that input-output pad. Some spherical input-output pads do not have a trace connected to that pad, such as a second input-output pad. The trace lines cannot be routed to the second input-output pad because a routing path does not exist due to minimum space requirements between the trace line and 1) neighboring trace lines as well as 2) input-output pads. Not enough routing space exists to route a trace line to that pad.
The spherical input/output pads are generally larger in width than the trace lines. This relationship may cause some trace lines, such as the first trace line, to be routed in off angles to maintain the minimum spacing requirements when routing around a given input/output pad to connect to its intended input/output pad. The routing of the trace lines in an off-angled shape effects the amount of trace lines that may be routed on a layer of a printed circuit board because of the minimum spacing requirements needed to be maintained between this trace line routed at an off angle and neighboring electrically conductive components. This spherical solder ball and input/output pad technology may reduce the routability of traces lines and the quantity of input/output pad actually connected to a trace line.
Typically, increasing the size of the package containing a printed circuit board and the amount of layers in a printed circuit board increases the number of input-output pads that can have a trace line routed to that pad. However, increasing the number of layers in a printed circuit board and the physical dimensions of that printed circuit board may increase the cost of the printed circuit board.
Also, as the physical size of some packages increases that package may exhibit co planarity issues, which can result in open joints. Either the package or the printed circuit board is not flat and level throughout its respective planar surface. Due to a height difference in the non co-planar surfaces, one or more of the solder balls does not physically connect to the surface of both the package and the printed circuit board.
A typical manufacturing process for securing a package to a printed circuit board is to use pre-formed solder spheres which are attached to the package substrate via a paste and ball attach. The package is then placed on the printed circuit board, which has pasted pads, and the entire assembly is reflowed providing a solder joint between the package and board.
The spherical solder ball typically placed on the input/output pad may provide an electro-mechanical connection to the package containing that printed circuit board. The solder ball of the input/output pad mates up with a solder connection on the package to make an electrical connection between the package and the printed circuit board. Also, the solder ball of the input/output pad mates up with that solder connection on the package to make a mechanical bond to secure the printed circuit board to the package in order to maintain the alignment of those two components. Decreasing pad size for some Ball Grid Array routing schemes may lead to solder fatigue and other solder joint reliability problems. The solder joint reliability problems may be exacerbated by coefficient of thermal expansion mismatches between the material forming the package and the materials forming the printed circuit board. Also, decreasing pitch spacing between trace lines may be an increased opportunity for solder shorts due to excess solder from the solder ball on the package and the solder paste on the printed circuit board.
While the invention is subject to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. The embodiments of the invention should be understood to not be limited to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.